Image-capturing devices, such as scanners, all-in-one devices, copiers, etc., sometimes use sheet feeders to feed media sheets, such as printed sheets, photographs, etc., to a scanning portion of the image-capturing device for scanning hardcopy images formed on the media sheets. Sheet feeders typically include a tray for receiving one or more media sheets, e.g., from a user. Some sheet feeders include a roller (e.g., sometimes called a pick roller) rotatably connected to an arm (e.g., sometimes called a pick arm) that is pivotally connected to the imaging device or a stationary portion of the sheet feeder, for example. When one or more media sheets are located in the tray, the arm overlies the media sheets so that the media sheets are interposed between the tray and the roller, with the roller contacting the uppermost media sheet.
The arm may be substantially parallel to the uppermost media sheet, e.g., when the tray is full of media sheets. However, when the tray is less than full, e.g., after a number of media sheets have been fed to the scanning portion, the arm is in a pivoted position relative to when the tray is full and forms an angle with the uppermost media sheet that is equal to the angular distance (e.g., the pivot angle) over which the arm has pivoted.
When torque is applied to the roller, the roller rolls relative to the arm and exerts a tangential force on a surface of the media sheet in contact therewith that causes the media sheet to move. The tangential force is substantially equal to the product of the coefficient of friction between the roller and the media sheet and the force exerted by the roller on the media sheet in a direction normal to the surface of the media sheet (e.g., commonly called the normal force) and perpendicular to the tangential force. It is often desirable to have substantially rolling contact, e.g., little or no slipping, between the roller and the media sheet as the media sheet moves, and, therefore, the coefficient of friction between the roller and the media sheet is substantially the coefficient of rolling friction.
The arm is at different pivot angles for different numbers of media sheets between the roller and the tray. However, the normal force exerted by the roller on the media sheet typically varies as the pivot angle changes, thus causing the tangential force exerted by the roller on the media sheet in contact therewith to change. For example, for some pivot angles, the normal force may result in a tangential force that insufficient to move the media sheet, e.g., the roller may slip relative to the media sheet. For other pivot angles, the normal force may result in a tangential force that is too high, e.g., causing several media sheets to move at once or causing damage to the arm, tray, roller, media sheets, and/or the imaging device.